1. Field of the Invention
The present invention relates generally to a method of developing an electrostatic latent image formed on an image carrying member and a developing device used for the development in an image forming apparatus such as a copying machine or a printer, and more particularly, to a developing device and a developing method in which a developer containing toners and carriers is conveyed to a developing area opposite to an image carrying member with it being held on the surface of a developer carrying member, and the toners in the developer are supplied to an electrostatic latent image formed on the image carrying member upon exerting an electric field between the developer carrying member and the image carrying member.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer, various developing devices have been conventionally used for developing an electrostatic latent image formed on an image carrying member.
As such a developing device, a developing device using a two-component developer containing toners and carriers has been used in addition to a developing device using a monocomponent developer containing only toners.
As the developing device using a two-component developer containing toners and carriers, a developing device as shown in FIG. 1 has been known.
In the developing device shown in FIG. 1, a magnet member 12 having a plurality of magnetic poles N, S, . . . is provided on the inner periphery side of a developer carrying member 11 in a cylindrical shape which is provided opposite to an image carrying member 1. A developer 2 contained in the main body 10 of the developing device is mixed and agitated by a developer agitating member 13, and is supplied to the developer carrying member 11 by the developer agitating member 13, to hold the developer 2 on the surface of the developer carrying member 11 by a magnetic force produced by the magnet member 12.
The developer carrying member 11 is rotated, to convey the developer 2 in the state of a magnetic brush, and the amount of the developer 2 conveyed by the developer carrying member 11 is adjusted by a regulating member 14, to convey the developer 2 in a suitable amount to a developing area opposite to the image carrying member 1 by the developer carrying member 11. A DC bias voltage is applied to the developer carrying member 11 from a DC power supply 15, and an AC bias voltage is applied from an AC power supply 16. An electric field which is an overlapping of an AC electric field and a DC electric field is exerted in a developing area where the developer carrying member 11 and the image carrying member 1 are opposite to each other, and the toners in the developer 2 are supplied to the image carrying member 1, thereby developing the electrostatic latent image formed on the image carrying member 1.
When the electric field which is an overlapping of the AC electric field and the DC electric field is thus exerted in the developing area where the developer carrying member 11 and the image carrying member 1 are opposite to each other to perform development, a high AC electric field must be exerted in the developing area in order to efficiently separate the toners in the developer 2 from the carriers. Particularly when toners having a small diameter are used, adhesion between the toners and the carriers is high. Accordingly, a higher AC electric field must be exerted.
When the AC electric field exerted between the developer carrying member 11 and the image carrying member 1 is thus strengthened, a leak occurs between a magnetic brush formed of the developer 2 on the surface of the developer carrying member 11 and the image carrying member 1. Further, the carriers in the developer 2 adhere to the image carrying member 1, so that noises are produced in a formed image.
In order to prevent the occurrence of a leak and the adhesion of carriers, it is considered that carriers having a high resistance are used for the developer 2. When the carriers having a high resistance are used, however, an electric field exerted between the developer carrying member 11 and the image carrying member 1 is weakened. Therefore, the toners in the developer 2 are not satisfactorily separated from the carriers. Accordingly, the toners are not sufficiently supplied to the electrostatic latent image formed on the image carrying member 1, so that the density of the formed image is decreased.